This invention relates to injection apparatus for die cast machines and more particularly to an improved fluid pressure control system for the injection apparatus.
To have better understanding of the invention a typical injection apparatus for a die cast machine will first be described with reference to FIG. 1 of the accompanying drawing, which diagrammatically illustrates a prior art fluid pressure control system for an injection cylinder of a die cast machine, not shown.
When a solenoid valve 3 is switched from position X to position Y, pressurized operating fluid, oil for example, from an accumulator ACC flows into the rear chamber 1 of a booster cylinder 8 through the restricted passage at position Y and a pipe 4. Then the operating fluid flows into the rear chamber 6 of an injection cylinder 7 via a check valve 13 in the booster cylinder 8 for advancing (toward left) the injection piston A at a low speed. Although not shown in the drawing, the righthand portion of the check valve 13 comprises a tube extending through booster piston B and containing a rod leading to a piston 16. The tube is provided with an opening for admitting the operating fluid into the interior of the booster piston B by opening the check valve 13. The operating fluid then enters into the rear chamber 6 through an opening, not shown, through the fore end wall of the booster piston B. When the injection piston A reaches a predetermined position while moving at the low speed, the solenoid valve 3 is switched from position Y to position Z so that the operating fluid from the accumulator ACC will enter into the rear chamber 6 of the injection cylinder 7 without passing through the restricted passage at position Y, thus advancing the injection piston at a high speed. During this high speed advancement of the injection piston A, the pressure of the operating fluid in the rear chamber 1 of the booster cylinder 8 and the rear chamber 6 of the injection cylinder 7 is relatively low due to the high speed movement of the injection piston A. For this reason, where the set pressure of a sequence valve 10 is selected to be higher than the pressure prevailing in said rear chambers at the time when the injection piston A is moving forwardly, the operating fluid in the fore chamber 2 of the booster cylinder 8 will be maintained at a sealed condition so that the booster piston B will not be advanced. As the injection piston A advances further so as to completely fill a metal mould, not shown, with molten metal or alloy, the load on the injection piston A increases, this condition being termed the "limit of pouring of the molten metal".
When the injection piston A reaches this limit while moving forwardly at high speed, the pressure of the operating fluid in the rear chamber 6 of the injection cylinder 7 rises, thereby closing the check valve 13. The pressure of the operating fluid in the rear chamber 1 of the booster cylinder 8 also rises to a predetermined pressure determined by the injection condition or the like. This increased pressure is transmitted to a sequence valve 10 through a pipe 4c thereby opening the sequence valve 10 and then opening a relief valve 9. As a consequence, the operating fluid in the fore chamber 2 of the booster cylinder 8 will be returned to a reservoir T via pipe 4d, valves 10 and 9, throttle valve 14, the fore chamber 15 of the injection cylinder 7 and a pipe 5. Accordingly, the booster piston B begins to move in the forward direction whereby the pressure of the operating fluid in the rear chamber 6 of the injection piston 7 is increased by the difference between the pressure receiving areas of front and rear surfaces of the booster piston B with the result that pressure is applied to the molten metal poured into the metal mould. A check valve 17 is provided for returning the booster piston to the original position.
FIG. 4 is an oscillogram showing the relationship between time (abscissa) and pressure (ordinate) prevailing while the injection cylinder of a die cast machine is operating. As shown, during the low speed stroke, the injection piston advances under a relatively low pressure whereas during the high speed stroke as the resistance against forward movement increases the pressure in the rear chamber 6 of the injection cylinder 7 increases somewhat above that of the low speed stroke. As the injection piston A reaches the limit of pouring of the molten metal it cannot advance further so that the pressure of the operating fluid in the rear chamber 6 of the injection cylinder 7 increases thus closing the check valve 13 in the booster piston B. The interval between the limit of pouring of the molten metal and the closure of the check valve 13 corresponds to an interval in which the pouring pressure increases.
When the check valve 13 in the booster piston B closes, the pressure of the operating fluid in the rear chamber 1 of the booster cylinder 8 increases so that the sequence valve 10 and the relief valve 9 are opened to discharge the operating fluid in the fore chamber 2 of the booster cylinder 8 into reservoir T through these valves. Accordingly, the booster piston B is permitted to advance thus compressing the operating fluid in the rear chamber 6 of the injection cylinder 7. The interval between this instant and the end of the pouring pressure increasing interval corresponds to the time lag of pressure increase. Thereafter, as the booster piston B advances, increased pressure of the operating fluid sealed in the rear chamber 6 of the injection cylinder 7 acts upon the injection piston A. The interval between the end of the time lag of pressure increase and an instant at which increase in the pressure acting upon the injection piston A ceases corresponds to the pressure increasing interval shown in the graph.
The sum of the pouring pressure increasing interval, the time lag of pressure increase and the pressure increasing interval is shown as a pressure build-up interval. The relationship between the pressure build-up and the time elapse during this interval has an important influence upon the quality of the die castings. More particularly, as the molten metal poured into the mould solidifies as the time elapses it is necessary to apply pressure to the mould as fast as possible. To this end, it is essential to greatly decrease the pressure build-up interval.
In the injection apparatus described above, when the injection piston A reaches the limit of pouring of the molten metal and cannot advance further, the pressure of the operating fluid in the rear chamber 6 of the injection cylinder 7 rises rapidly, thus closing the check valve 13 in the booster piston B. Then, the pressure in the rear chamber 1 of the booster cylinder 8 rises to open the sequence valve 10 and the relief valve 9 thus advancing the booster piston B by discharging the operation fluid in the fore chamber 2 of the booster cylinder 8 through valves 10 and 9 so that the time lag of pressure increase is considerably large.
Furthermore, for the purpose of increasing or decreasing the pressure increasing interval shown in FIG. 4 in accordance with the configuration, or thickness of the die castings, the rate of discharge of the operating fluid in the fore chamber 2 of the booster cylinder 8 into reservoir T is controlled by the throttle valve 14. However, with the apparatus described, to decrease further the pressure increasing interval it is necessary to decrease the flow resistance of various pipe lines as well as the resistance to the operating fluid in the fore chamber 2 of the booster cylinder 8. In addition, as it is necessary to provide valves for passing a large quantity of the operating fluid, the cost of installation increases.